Known from the European patent No. EP1612403 is a fuel injector for an internal-combustion engine comprising:                a casing, ending with a nozzle for injecting fuel into a corresponding engine cylinder;        a movable needle for closing and opening the nozzle;        a rod, housed in the casing and slidable along its own axis for controlling the movement of the needle; and        a metering servovalve, housed in the casing and comprising:a) an actuator;b) a control chamber, which communicates with an inlet for the fuel and with a passage for outlet of the fuel, which has a calibrated portion and the pressure of which controls axial sliding of the rod;c) an open/close element, defined by a sleeve that is axially movable under the action of the actuator between a closing position, in which it closes the discharge channel, and an opening position, in which it leaves the discharge channel open, so as to vary the pressure in the control chamber in order to close and open the nozzle; andd) an axial stem, placed in a position fixed with respect to the casing and having an outer lateral surface, through which the discharge channel exits.        
The sleeve is mounted on the outer lateral surface of the axial stem in an axially slidable and substantially fluid-tight way and, in the closing position, closes the discharge channel so as to be subjected to a zero axial resultant by the pressure of the fuel. In said system, where the metering servovalve and the open/close element defined by the sleeve are of the so-called “balanced” type, the forces required of the actuator, and, consequently, the overall dimensions, are small. In particular, even with minor lifts of the open/close element, it is possible to obtain large passage cross-sections of the fuel, with consequent advantages in the dynamic behaviour of the injector, i.e., without any phenomena of so-called “rebound” of the open/close element at the end of the travel of opening and closing.
The metering servovalve has a so-called valve body made up of three pieces, namely, a tubular guide body, which defines laterally the control chamber and axially guides the rod, a distribution body comprising the axial stem, and a disk, which is placed axially between the tubular guide body and the distribution body and has the aforesaid calibrated portion made axially.
The known solution just described is far from satisfactory in so far as it is relatively complex to produce in a precise way to guarantee tightness in regard to the fuel that flows from the control chamber into the outlet passage. In fact, said known solution requires grinding operations on as many as four fluid-tight metal surfaces, i.e., on the surfaces in the areas of axial coupling between the distribution body and the disk and between the disk and the tubular guide body.
In addition, the theoretical average diameter in which fluid tightness between the tubular guide body and the disk is achieved is relatively large so that the pressure acting on a surface having said diameter also causes large axial forces, with the consequent risk of considerable deformations, in particular on the disk. Said deformations, on the one hand, cause errors in the lift of the open/close sleeve with respect to what is envisaged in the design stage and, on the other hand, tend to cause a further increase in the theoretical average diameter of fluid tightness between the deformed disk and the tubular guide body, hence causing progressive deterioration of the situation.
In addition, the operations of machining, handling, and assembly of the three pieces that constitute the valve body are considerably long and costly. In order to attempt to overcome these drawbacks, it is known, for example from the European patent No. EP1621764di, to make the valve body of the metering valve of a single piece.
This solution entails extremely high costs to guarantee very strict geometrical tolerances, in particular as regards the shaft of the axial stem, which must coincide (with tolerances of the order of a micron) with the axis of the blind hole in which the rod is guided.
In addition, once again in the case of a valve body made of a single piece, it is extremely complex to produce more than two calibrated portions placed in series to one another, in so far as it would be necessary, in theory, to force at least two additional inserts in an axial direction operating through the blind hole in which the rod is guided.